Semiconductor devices such as thyristors and transistors, for example field-effect controlled switching devices such as a Metal Oxide Semiconductor Field-effect Transistor (MOSFET) or an Insulated Gate Bipolar Transistor (IGBT) have been used for various applications including but not limited to switches in power supplies and power converters, electric cars, air-conditioners, and even stereo systems. Particularly with regard to power devices capable of switching large currents and/or operating at higher voltages, low on-state resistance, which is subsequently referred to as on-resistance Ron, soft switching behavior (soft-recovery) and high voltage blocking capability are often desired. The softness of a semiconductor device may be described in terms of overvoltages and/or voltage oscillations and/or current oscillations occurring during switching-off the semiconductor device.
In an IGBT, an isolated gate FET (Field Effect Transistor) is used for control of a bipolar transistor. In so doing, the low on-resistance Ron and the fast voltage control of the isolated gate FET is combined in a single semiconductor device with the high current and low saturation voltage VCEsat of the bipolar transistor. Accordingly, IGBTs are widely used in medium to high-power applications such as switching mode power supplies, inverters and traction motor controls. A single power IGBT may have a current switching capability of up to about 100 A or more and may withstand blocking voltages of up to 6 kV or even more.
When an IGBT is switched-off (turned-off), the current may retroact on the gate. This may result in oscillations and may even reduce the controllability. To improve the softness of IGBTs, the backside emitter efficiency may be laterally varied. This may however favor inhomogeneous current distributions.
Furthermore, deep vertical trench gates are often used for power IGBTs. In particular the bottom portion of the gate oxide may be exposed to high static electric fields, for example electric fields resulting from trapped charges in the gate oxide, and/or high dynamic electric fields in a blocking mode of the IGBT. This may result in reducing the blocking capability and even in a permanent device failure. Further, hot charge carriers formed in an avalanche event during the blocking mode may be injected into the gate oxide. This may result in shifting the operating points of the device and/or the location of the breakthrough. In particular, the so-called dynamic avalanche, in which—compared to the static case—the electric field gradient during the blocking mode gets substantially steeper due to flowing holes added to the background doping, may cause oscillations and/or device failure.
Known measures to reduce oscillations during switching-off IGBTs have only a limited effect and/or are accompanied by side-effects. For example, the bottom portion and/or side portions of the gate oxide may be made thicker. However, the thickness of the side portions of the gate oxide is typically limited to half of the trench width. Further, mechanical stress typically increases with the thickness of the gate oxide. Using so called high-k materials such as hafnium oxide having a larger dielectric constant than silicon oxide as gate oxide and gate dielectric, respectively, typically only result in a limited reduction of the oscillations and may result in a more complex manufacturing. Alternatively, additional trenches on source potential (inactive gate trenches) may be arranged between the trenches on gate potential. During switching-off, the hole current may flow below the additional trenches towards emitter contacts. Thus, the impact of the hole current on the gates may effectively be reduced. However, a gate resistance, which is too high for many applications, may be required to influence the switching-off. Further, this design typically uses a large device area.